Numerous methods and apparatuses are known for the purpose of measuring density and density variations in various substances. One group of this prior art employs visible light waves and their various properties such as phase shifts, interferometric fringe patterns, diffraction, and refraction. In another group, electromagnetic waves other than visible light (e.g., x-rays and infrared) have been used to make measurements of density. Prior art also teaches the use of a special birefringement crystal phase plate (phase shifting assembly) to encode the angle of incidence as a poralization phase shift. Prior art then uses the phase-shift-to-angle-of-incidence relationship to locate the position or source of an incoming electromagnetic wave. However, prior art does not teach, and does not make obvious, the application of this phase shifting assembly to encode as a poralization phase shift the change in the angle of incidence that would result from the refractive effects of density variations in a substance. This new use of the phase shifting plates establishes a known and definable relationship between poralization phase shifts and the density variations (i.e., density variation causes refraction, which causes a change in the angle of incidence, which is encoded as a poralization phase shift).
The use of a special phase shifting assembly to record an incoming angle of incidence and encode that angle of incidence as a poralization phase shift is well documented in U.S. Pat. No. 4,626,100 issued Dec. 2, 1986; U.S. Pat. No. 4,624,563 issued Nov. 25, 1986; U.S. Pat. No. 5,191,392 issued Mar. 2, 1993, and U.S. Pat. No. 6,348,998 issued Feb. 19, 2002. An important distinction between these special phase shifting assemblies and prior art is that these phase shifting assemblies measure the phase shift between the ordinary and extraordinary rays of a single wave. Prior art recognizes a phase shift from interference patterns or another related characteristic of light and requires at least two waves. Prior applications of these special phase shifting assemblies have been to locate the position of a source of light relative to the assembly, to generate a Fourier transform of the incident intensity image, and to generate high quality fringe patterns that vary in number with adjustments to a certain twist angle of the apparatus.
One objective of prior art is to adjust for the effects that density variations in gas have on the use of interferometric measurement methods. U.S. Pat. No. 5,404,222 issued Apr. 4, 1995, discloses and discusses a method of compensating for the refractive effects that turbulent gas has on interferometric measurements. All such prior art, including U.S. Pat. No. 5,404,222, appears dependent upon measuring the effects of the density variations by comparing two waves. Generally, one wave that passes through the varying density substance is compared with a second wave that has not passed through that substance to calculate the effects of the density variations. The differences in the two waves (e.g., path length or interference patterns caused by a phase shift) are then compared to determine the refractive index, which is then used to determine the density difference.
The concept of an X-ray interferometer was the subject of U.S. Pat. No. 2,999,931 issued Sep. 12, 1961 and U.S. Pat. No. 6,195,410 issued Feb. 27, 2001. In these patents the interference patterns created by the specific method or apparatus are used to obtain the desired information.
While the interference patterns discussed in the above references also can be used to determine the phase shift that created those patterns, measuring a phase shift between the ordinary and extraordinary rays of a single wave is not taught by the references other than those utilizing the phase shifting assembly. Therefore, prior art, individually and in combination, does not teach and does not make obvious the application of this phase shifting assembly to encode as a poralization phase shift the change in the angle of incidence that would result from the refractive effects of density variations in a substance.
The necessity of an unaffected or reference wave for comparison with the wave affected by the density varying substance is a requirement which may not be: available; timely available; or otherwise convenient in all situations. The need for real-time, accurate measurements of the density (static or dynamic) of a substance is an objective for which prior art cannot guarantee a workable or practical solution in all situations.
The method claims of this invention represent a unique combination of steps that is not taught, motivated or suggested by the prior art. The apparatus claims of this invention represent a unique combination of means and means plus structure that is not taught, motivated or suggested by the prior art. Certain apparatus claims include the use of a phase shifting assembly to perform the encoding means. Several different phase shifting assemblies have been noted in the references above. These claims represent a new use of those assemblies.
Described herein are a method and an apparatus for measuring the density variations, static and dynamic, in substances that are at least partially transparent to electromagnetic waves. A typical application would be to visually display the real-time variations in densities within a gas or liquid. A previously patented method using a special birefringement crystal phase shifting assembly encodes the angle of incidence resulting from the refractive effects of the electromagnetic waves having passed through a density variation. The angle of incidence is encoded as a poralization phase shift. Specifically, the poralization phase shift is between the ordinary and the extraordinary rays. That poralization phase shift has a known and definable relationship to the gas density experienced by the electromagnetic wave during its path.